The present invention relates to cable modem networks, and more particularly to input power estimation methods that compensate for variations in tuner gain characteristics.
Digital modems are increasingly being used by consumers as cable modem network service providers obtain additional subscribers. Cable modem devices designed for the mass-market are designed to be affordable to as many consumers as possible. However, low-cost, high production devices manufactured for the mass consumer market often exhibit variations and irregularities in their operating characteristics, such as gain non-linearities, frequency ripple and temperature effects. These variations make it difficult to measure the radio frequency (RF) input power to the cable modem accurately.
Currently known calibration solutions do not address variations in the tuner gain and intermediate frequency (IF) amplifier gain in the tuner of the cable modem as the frequency and power level of the input signal changes. Further, any calibration method that applies the same parameters globally to all cable modem devices does not consider the fact that individual devices may exhibit slightly different operating characteristics and may have different irregularities. These variations may adversely affect the cable modem""s performance if not adequately addressed via an accurate input RF power determination.
Although it is theoretically possible to use the cable modem to calculate data that will compensate for variations in tuner gain characteristics, this would require incorporating additional calculation circuitry into the cable modem, increasing the cable modem""s complexity and generating a device that is likely to be too expensive for the mass consumer market.
There is a need for a simple method and system to correct for variations and irregularities that are commonly encountered in low-cost, high volume cable modems when estimating input power to the cable modem.
Accordingly, the present invention is directed to a method for estimating input power to a cable modem device having a tuner and a modem. The method includes generating a look-up table that contains look-up values used to compute an estimated input power to the modem""s receiver. The look-up table is stored in the modem for reference. The method includes inputting a plurality of calibration signals having known input frequencies and known input power levels into the device""s receiver, recording a calibration point corresponding to each of the calibration signals in a calibration matrix, and interpolating the calibration points in said calibration matrix to generate the look-up table values.
A cable modem device according to the claimed invention has the look-up table stored in the modem, preferably as 8-bit data. During modem operation, the modem checks the frequency of an input signal received by the modem""s receiver, checks an accumulated value of an integrator, checks the look-up table for the look-up value corresponding to the frequency and accumulated integrator value, and uses the look-up value to determine an estimated input power. Because the look-up table values are derived from the cable modem device""s actual operating characteristics, the input power estimate will reflect any variations or irregularities in the specific device being used.